A power factor is expressed by power/(current×voltage) and called a phase difference between current and voltage of an alternating current (AC) circuit.
In a capacitor input-type converter, a phase difference occurs when a waveform is distorted and a harmonic content is included in an input current. For this reason, there is used a power factor improving converter that controls a switching current to follow a waveform of a ripple voltage to improve a power factor without using a smoothing capacitor on a primary side where the ripple voltage is supplied.
As the power factor improving converter, there is a current control type that controls the switching current in a critical mode.
The power factor improving converter of the current control type includes a boost chopper circuit wherein a converter is configured using, for example, an inductor, a switching element, a diode and an output capacitor.
The power factor improving converter turns on the switching element to cause current to flow into the inductor and stores energy to the inductor. When the switching current reaches a target level that is set based on a supplied ripple voltage and an output voltage, the power factor improving converter turns off the switching.
When the switching element turns off, the current is supplied from the inductor to the output capacitor through the diode to release the energy stored in the inductor. The power factor improving converter detects that the current flowing into the inductor reaches a critical point (a current value reaches zero) and turns on the switching element again.
The power factor improving converter repeats such operations to maintain the voltage of the output capacitor provided at the output side constant. At the same time, a current waveform due to the supplied ripple voltage follows the waveform of the ripple voltage to form a sine wave, thereby improving the power factor.
However, since the above conventional power factor improving converter performs control in such a way that the output voltage becomes constant, when an alternating current supplied from an AC power source reduces to AC 100V from AC 200V on an effective value, a boost ratio (ratio between the output voltage and the ripple voltage) increases, so that boost energy becomes high. Particularly, when the output current on a secondary side is high, switching loss due to boosting is increased to reduce efficiency.